Method of producing a bonded wafer and the bonded wafer

ABSTRACT

There is provided a method of producing a bonded SOI wafer wherein a silicon single crystal ingot is grown according to Czochralski method, the single crystal ingot is then sliced to produce a silicon single crystal wafer, the silicon single crystal wafer is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100° C. to 1300° C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700° C. to 1300° C. for one minute or more without cooling the wafer to a temperature less than 700° C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, and the resultant wafer is used as the bond wafer, and a bonded SOI wafer produced by the method. There can be provided a SOI wafer that has a SOI layer having few crystal defects, good surface roughness and high quality in high productivity, in high yield and with low cost.

TECHNICAL FIELD

[0001] The present invention relates to a method of producing a bondedwafer having very few crystal defects on and near the surface and abonded SOI wafer.

BACKGROUND ART

[0002] SOI (silicon on insulator) has a buried oxide film (BOX: BuriedOXide) as a insulator film right below a silicon layer that is to be aregion for fabrication of device, and is expected to be a siliconmaterial for high speed device with electric power saving performance.One of methods for producing a SOI wafer is a bonding method wherein twosilicon single crystal wafers, one of which is to be a bond wafer (asubstrate that is to be a SOI layer on which a device is fabricated),and the other of which is to be a base wafer (a substrate supporting theSOI layer) are bonded via a oxide film, and then thickness of the bondwafer is decreased to form a SOI structure. The method has an advantagethat crystallinity of the SOI layer is excellent, and insulatingproperty of BOX is high, but has a disadvantage that quality of the SOIlayer is influenced much by quality of the bond wafer.

[0003] Specifically, it has been known that there exist micro crystaldefects (Grown-in defects) such as COP (Crystal Originated Particles)that is a void type defect or the like in a silicon wafer producedaccording to Czochralski method, which adversely affects characteristicsof the device such as oxide dielectric breakdown voltage. In order tosolve the problem, there have been known that visible defects can bereduced by using, as a substrate for a bond wafer, a wafer wherein a CZwafer is subjected to annealing in a hydrogen atmosphere or an epitaxialwafer wherein an epitaxial layer is formed on a CZ wafer (See JapanesePatent Application Laid-open (kokai) No. 9-22993 and Japanese PatentApplication Laid-open (kokai) No. 9-260619).

[0004] However, two heat treatments, namely heat treatment such ashydrogen annealing or epitaxial growth and heat treatment for forming aburied oxide film, which may lead to increase of cost and lowering ofthrough put.

[0005] In the case of the epitaxial wafer, haze (surface roughness) isgenerated on the surface of the epitaxial layer, or projection calledmound is sometimes formed. They may cause bonding failure when thewafers are bonded. Accordingly, it is sometimes necessary to polish thesurface of the epitaxial layer before bonding in that case.

[0006] On the other hand, crystal defects are reduced by hydrogenannealing only at layer quite near the surface (about 0.5 μm from thesurface), and thus, if a SOI wafer having a thickness more than thevalue is produced, an area where crystal defects are not reduced isexposed. Therefore, crystal defects in the whole SOI layer cannot bereduced, unless any measures are taken, for example, further hydrogenannealing is conducted after SOI wafer is produced. Furthermore,according to annealing with hydrogen, quartz tube, a boat made of SiC orthe like are always etched, and contamination with metal impurities orthe like are caused thereby.

[0007] Furthermore, when heat treatment is conducted in a hydrogenatmosphere, it is necessary to take out the wafer after replacing theatmosphere in the heat treatment furnace with nitrogen gas for safety.At that time, the surface of the wafer is locally etched with slightamount of oxygen and water vapor contained in nitrogen gas, which maydegrading surface roughness such as haze or the like, which may lead tobonding failure when they are bonded.

[0008] Recently, it has been reported that there can be produced CZwafer wherein Grown-in defects are significantly reduced if crystal ispulled with strictly controlling a growth rate and temperature gradientof solid-liquid interface while single crystal is grown according toCzochralski method. It can be easily expected that SOI wafer having fewdefects in SOI layer can be produced if such a wafer is used as a bondwafer. However, if the crystal is pulled under such significantly strictgrowing condition may naturally lead to lowering in yield, resulting insignificant increase of cost for production.

[0009] On the other hand, single crystal produced according to FZ methodhas no COP defects as observed in CZ single crystal, but FZ crystalhaving a diameter more than 150 mm cannot be produced at commerciallevel. Although FZ crystal having a diameter of 200 mm can be producedat experimental level, there is no hope for producing a large diameterwafer having a diameter of 300 mm, 400 mm in the future.

DISCLOSURE OF THE INVENTION

[0010] The present invention has been accomplished to solve theabove-mentioned problems. A main object of the present invention is toprovide a SOI wafer that has a SOI layer having few crystal defects andhigh quality in high productivity, in high yield and with low cost byusing a wafer wherein grown-in defects in a surface-layer part ofsilicon single crystal wafer produced by CZ method are eliminated orreduced effectively by heat treatment as a bond wafer of a bonded wafer.

[0011] To achieve the above mentioned object, the present inventionprovides a method of producing a bonded SOI wafer comprising bonding abond wafer and a base wafer via an oxide film and then reducingthickness of the bond wafer, wherein a silicon single crystal ingot isgrown according to Czochralski method, the single crystal ingot is thensliced to produce a silicon single crystal wafer, the silicon singlecrystal wafer is subjected to heat treatment in a non-oxidizingatmosphere at a temperature of 1100° C. to 1300° C. for one minute ormore and continuously to a heat treatment in an oxidizing atmosphere ata temperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, and the resultant wafer is used as the bond wafer.

[0012] As described above, if the wafer produced according toCzochralski method is subjected to heat treatment in a non-oxidizingatmosphere at a temperature of 1100° C. to 1300° C. for one minute ormore and continuously to a heat treatment in an oxidizing atmosphere ata temperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, and the resultant wafer is used as the bond wafer, asilicon single crystal wafer having high quality wherein Grown-indefects near the surface of the wafer that are harmful for fabricationof semiconductor device can be eliminated or decreased in short time canbe used as a bond wafer, so that SOI wafer that has a SOI layer havingfew crystal defects and high quality can be produced in highproductivity, in high yield with low cost.

[0013] The present invention also provides a method of producing abonded SOI wafer comprising bonding a bond wafer and a base wafer via anoxide film and then reducing thickness of the bond wafer, wherein asilicon single crystal ingot is grown according to Czochralski method,the single crystal ingot is then sliced to produce a silicon singlecrystal wafer, the silicon single crystal wafer is subjected to heattreatment in a non-oxidizing atmosphere at a temperature of 1100° C. to1300° C. for one minute or more and continuously to a heat treatment inan oxidizing atmosphere at a temperature of 700° C. to 1300° C. for oneminute or more without cooling the wafer to a temperature less than 700°C. to provide a silicon single crystal wafer wherein a silicon oxidefilm is formed on the surface, at least one of hydrogen ions and raregas ions are implanted into the surface via a silicon oxide film of thewafer to form an ion implanted layer, and the resultant wafer is used asthe bond wafer, which is then brought into close contact with the basewafer via the silicon oxide film of the bond wafer, followed bydelamination at the ion implanted layer by heat treatment.

[0014] As described above, in method of producing a bonded SOI wafer, byusing the method wherein the wafer produced according to Czochralskimethod is subjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, at least one of hydrogen ions and rare gas ions areimplanted into the surface via a silicon oxide film of the wafer to forman ion implanted layer, and the resultant wafer is used as the bondwafer, which is then brought into close contact with the base wafer viathe silicon oxide film of the bond wafer, followed by delamination atthe ion implanted layer by heat treatment (so called ion implantationdelamination method), a silicon single crystal wafer having high qualitycan be used as a bond wafer, and surface condition of the SOI waferafter delamination is good, so that SOI wafer having excellent thicknessuniformity can be produced by a relatively easy method.

[0015] In that case, the bond wafer delaminated at the ion implantedlayer in the above-mentioned method of producing a bonded SOI wafer ofthe present invention can be used as a new bond wafer.

[0016] As described above, in the bond wafer delaminated at the ionimplanted layer in the method of producing a bonded SOI wafer of thepresent invention, grown-in defects in zone at a depth of about 5 to 10μm or more from the surface are eliminated, and thickness of the thinfilm delaminated at the ion implanted layer is about one μm at thickest,so that the bond wafer has denuded (low-defect) zone with sufficientdepth, even though it is a wafer after delamination of a thin film.Accordingly, even if the surface of the wafer is polished for reuse,sufficient denuded zone remains. Therefore, if it is used as a new bondwafer, and bonded to the base wafer via the oxide film, and thickness ofthe bond wafer is decreased to produce a SOI wafer, it is not necessaryto conduct further heat treatment of the bond wafer before bonding forelimination of grown-in defects. Thereby, a bonded SOI wafer having highquality can be produced efficiently.

[0017] Furthermore, the bond wafer delaminated at the ion implantedlayer in the above-mentioned method of producing a bonded SOI wafer ofthe present invention can be used as a new base wafer.

[0018] At an inner part (a bulk part) than denuded zone near the surfaceof the bond wafer after delamination of a thin film, a lot of oxideprecipitates are sometimes generated due to influence of heat treatment.In that case, if the wafer is used as a new base wafer, and bonded tothe bond wafer via the oxide film, and thickness of the bond wafer isdecreased to produce a SOI wafer, the SOI wafer having high performancein gettering of heavy metal impurities or the like can be obtained. Inthat case, even though a lot of oxide precipitates are generated in abulk part, a surface-layer part is denuded zone as described above, sothat oxide precipitates are never exposed on the surface of the basewafer, and there is no adverse effect to bonding with a bond wafer.

[0019] The above-mentioned non-oxidizing atmosphere is preferably argon,nitrogen or a mixed gas of argon and nitrogen.

[0020] Because, the atmosphere of argon, nitrogen or a mixed gas ofargon and nitrogen can be easily handled and inexpensive.

[0021] The above-mentioned oxidizing atmosphere may contain water vapor.

[0022] As described above, if the oxidizing atmosphere contains watervapor, an oxidation rate is high, and defects can be eliminatedefficiently in quite short time by injection of interstitial silicon.Since the oxide film formed on the bond wafer gets relatively thick, itis suitable for production of SOI wafer having a thick BOX.

[0023] In that case, the above-mentioned oxidizing atmosphere can be dryoxygen atmosphere or a mixed gas atmosphere of dry oxygen and argon ornitrogen.

[0024] As described above, if the oxidizing atmosphere is dry oxygenatmosphere or a mixed gas atmosphere of dry oxygen and argon ornitrogen, a growth rate of the oxide film is low, and thickness of theoxide film formed on the surface of the bond wafer after heat treatmentcan be made thin, and thus, it is suitable for production of the SOIwafer having thin BOX.

[0025] The thickness of the oxide film formed by the above-mentionedheat treatment in the oxidizing atmosphere is preferably 20 to 100 nm.

[0026] As described above, if the thickness of the oxide film formed bythe above-mentioned heat treatment in the oxidizing atmosphere is morethan 20 nm, COP at a surface-layer part of the bond wafer can be removedsufficiently. If the thickness is 100 nm or less, time necessary for thestep can be short even in the case that the formed oxide film needs tobe removed. Furthermore, in the case that the SOI wafer is producedaccording to the above-mentioned ion implantation delamination method,thickness uniformity of the SOI layer gets better, since an absolutevalue of the deviation in thickness of the oxide film on the surfacegets small.

[0027] The oxide film can be previously formed on the surface of thewafer before the heat treatment in a non-oxidizing atmosphere.

[0028] If such an oxide film is previously formed, the surface of thewafer can be protected so that formation of thermal nitride film on thesurface of the wafer due to heat treatment or surface roughness due toetching can be prevented. Therefore, bonding failure when the wafers arebonded can be prevented.

[0029] In that case, thickness of the thermal oxide film on the surfaceof the wafer after the above-mentioned heat treatment in the oxidizingatmosphere is preferably 300 nm or more.

[0030] As described above, if the thermal oxide film having a thicknessof 300 nm or more is grown, COP on the surface of the wafer can beeliminated by reflow phenomenon of silicon oxide during growth of theoxide film even when the oxide film is previously formed on the surfaceof the wafer before conducting the heat treatment in the non-oxidizingatmosphere, so that COP on the surface of the wafer can be eliminatedmore surely.

[0031] A silicon single crystal ingot is preferably grown according toCzochralski method with controlling a cooling rate at 1150° C. to 1080°C. of the single crystal ingot to be 2.3° C./min or more.

[0032] As described above, if a silicon single crystal ingot is grownaccording to Czochralski method with controlling a cooling rate at 1150°C. to 1080° C. of the single crystal ingot to be 2.3° C./min or more, asize of grown-in defect gets small. Since the above-mentioned heattreatment is conducted to such a wafer, grown-in defects in asurface-layer part of the wafer can be eliminated or reduced moreefficiently. Accordingly, a SOI wafer having a SOI layer with higherquality can be produced in high productivity.

[0033] In that case, it is preferable that a silicon single crystalingot in which nitrogen is doped is grown according to Czochralskimethod.

[0034] As described above, if a silicon single crystal ingot in whichnitrogen is doped is grown according to Czochralski method, the size ofgrown-in defect becomes smaller by nitrogen doping. Further, and theheat treatment is conducted thereto, and thus grown-in defects in asurface-layer part of the wafer can be more efficiently eliminated orremoved. Accordingly, a SOI wafer having a SOI layer with higher qualitycan be obtained in high productivity.

[0035] In that case, when growing silicon single crystal ingot in whichnitrogen is doped according to Czochralski method, the concentration ofnitrogen doped in the single crystal ingot is preferably 1×10¹⁰ to5×10¹⁵ atoms/cm³.

[0036] Because, 1×10¹⁰ atoms/cm³ or more is preferable in order tosuppress growth of grown-in defects sufficiently, and 5×10¹⁵ atoms/cm³or less is preferable in order not to prevent formation of singlecrystal of silicon single crystal.

[0037] Furthermore, when the silicon single crystal ingot is grownaccording to Czochralski method, the concentration of oxygen containedin the single crystal ingot is preferably 18 ppma (JEIDA: JapanElectronic Industry Development Association) or less.

[0038] If oxygen concentration is low as described above, growth ofcrystal defects can be suppressed further, and formation of oxideprecipitates at a surface layer can also be prevented.

[0039] A bonded SOI wafer produced according to the method of thepresent invention is, for example, a bonded SOI wafer wherein a SOIlayer consists of CZ silicon single crystal wafer, thickness of the SOIlayer is 5 μm or less, and 1.3 number/cm² or less of COP having a sizeof 0.09 μm or more exist at any region in depth direction of the SOIlayer.

[0040] As described above, in the bonded SOI wafer of the presentinvention, there exist very few COP at any region in depth direction ofSOI layer, even if thickness of SOI layer is more than 0.5 μm.Furthermore, SOI wafer of the present invention does not need to besubjected to hydrogen anneal or the like after production of SOI wafer,and thus productivity is also high.

[0041] According to the present invention, it is possible to eliminatevoid defects in deeper region efficiently compared with conventionalmethods, and therefore SOI layer with high quality can be formed.Furthermore, since heat treatment in non-oxidizing atmosphere and heattreatment in oxidizing atmosphere can be conducted in the same batch,the number of the steps in production of SOI does not increase, and thuscost therefor does not increase either. Furthermore, since heattreatment can be conducted without using hydrogen, heat treatment can beconducted with no danger of contamination from the furnace due tohydrogen and explosion. Furthermore, since CZ wafer is used, it can beapplied to a wafer having a large diameter as 300 mm or more.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a flow chart showing one example of production processesof a bonded SOI wafer according to the present invention.

[0043]FIG. 2 is a view showing outline of heat treatment applied to asilicon single crystal wafer to be a bond wafer before bonding.

[0044]FIG. 3 is a graph showing number of COP in the wafer after heattreatment in Example 1, Comparative Example 1 and Comparative Example 2.

[0045]FIG. 4 is a graph showing TZDB good chip yield in the wafer afterheat treatment in Example 1, Comparative Example 1 and ComparativeExample 2.

[0046]FIG. 5 is a graph showing TDDB good chip yield in the wafer afterheat treatment in Example 1, Comparative Example 1 and ComparativeExample 2.

[0047]FIG. 6 is a graph showing number of COP on the surface of thewafer after heat treatment in Examples 1 to 3.

[0048]FIG. 7 is a graph showing TZDB good chip yield in the SOI wafer ofExample 4.

[0049]FIG. 8 is a graph showing TDDB good chip yield in the SOI wafer ofExample 4.

[0050]FIG. 9(a) is a graph showing a relation between oxygenconcentration in annealing atmosphere and number of COP, a FIG. 9(b) isa graph showing a relation between a thickness of oxide film formed byannealing and number of COP.

[0051]FIG. 10 is a graph showing comparison of transition ofcontamination level of metal impurities when each of heat treatment ofthe present invention and conventional invention is conducted indifferent tubes repeatedly.

BEST MODE FOR CARRYING OUT THE INVENTION

[0052] The embodiment of the present invention will be further describedbelow, but is not limited thereto.

[0053] The inventors have found that grown-in defects on the surface andin a surface-layer part of the wafer can be eliminated or reduced, andsurface roughness of the wafer can be improved by producing a siliconwafer according to Czochralski method and then subjecting the wafercontinuously to heat treatment at high temperature in non-oxidizingatmosphere, especially in atmosphere of argon, nitrogen, or mixedatmosphere thereof, and oxidation heat treatment at high temperature inoxidizing atmosphere, and that a SOI wafer having an excellent SOI layercan be produced in high productivity by using the silicon wafer as abond wafer of a bonded SOI wafer, and have studied condition further,and thereby completed the present invention.

[0054] As described above, in order to eliminate or reduce, grown-indefects on the surface and in a surface-layer part of the wafer, a waferused generally in commercial level is a wafer grown at a general crystalgrowth rate of about 1.0 mm/min or more and subjected to heat treatmentat high temperature in hydrogen atmosphere to eliminate grown-indefects. The method has already been used for actual production ofdevices, but defects still remains in a surface-layer part of the wafer(for example, 0 to 5 μm from the surface).

[0055] Reasons therefor have been considered as follows. Two steps arenecessary in order to eliminate grown-in defects that are aggregationsof atomic vacancy. Namely, they are a step of melting an inner walloxide film of defects, which prevents true point defects from changinginto grown-in defects, and a subsequent step of filling the grown-indefect with interstitial silicon.

[0056] In the heat treatment at high temperature in hydrogen atmosphere,it is considered that melting of an inner wall oxide film of grown-indefects in a surface-layer part of the wafer can be efficiently causedby significant oxygen out-diffusing effect. However, filling of grown-indefect with interstitial silicon cannot be caused efficiently, sinceboth of interstitial silicon that is a Schottky defect and atomicvacancy are injected from the surface of the wafer under heat treatmentat high temperature in hydrogen atmosphere.

[0057] Accordingly, it takes long time for a step of filling grown-indefects with interstitial silicon in heat treatment at high temperaturein hydrogen atmosphere. Especially, in order to eliminate grown-indefect having a size of 150 nm or more in terms of their diameter, it isnecessary to perform a heat treatment at high temperature as 1200° C.for long time as 5 hours or more.

[0058] It lowers productivity of the wafer, and is not preferable from apoint of safety, since it requires heat treatment at high temperature inhydrogen atmosphere for long time. Furthermore, since heat treatment athigh temperature for long time is conducted, oxygen precipitation nucleiin silicon single crystal wafer are also eliminated, gettering effect ofheavy metal that is effective for device process are also lost.

[0059] The inventors solved the above problems by conducting heattreatment in a non-oxidizing gas containing hydrogen less than explosionlimit (about 4%), especially atmosphere of argon, nitrogen or mixturethereof, at a temperature of 1100° C. to 1300° C. for one minute or moreand continuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. Namely, a step ofmelting of an inner wall oxide film of defects is efficiently conductedby heat treatment at high temperature in atmosphere of argon, nitrogenor a mixed gas thereof, and then heat treatment in oxidizing atmosphereis continuously conducted to progress efficiently a step of fillinggrown-in defect with interstitial silicon.

[0060] The step of melting of an inner wall oxide film of defects isefficiently conducted by heat treatment at high temperature inatmosphere of non-oxidizing gas containing hydrogen less than explosionlimit (about 4%), especially, argon, nitrogen or a mixed gas thereof,since it is quite difficult from the point of safety that heat treatmentin oxygen atmosphere is continuously conducted after heat treatment athigh temperature in hydrogen atmosphere. Two steps of heat treatment canbe continuously safely conducted only by using atmosphere ofnon-oxidizing gas containing hydrogen less than explosion limit (about4%) especially, argon, nitrogen or a mixed gas thereof, instead ofhydrogen atmosphere. Moreover, due to oxygen out-diffusion effect,melting of inner wall oxide film of grown-in defects is conductedefficiently in heat treatment at high temperature in atmosphere ofnon-oxidizing gas, especially argon, nitrogen or mixed gas thereof, aswell as it is conducted in hydrogen atmosphere.

[0061] It is considered that oxygen out-diffusion effect in argonatmosphere is equivalent to that in hydrogen atmosphere, since naturaloxide film on the surface of the wafer is sublimated as SiO gas andremoved in heat treatment at high temperature as 1100 to 1300° C. inargon atmosphere. In nitrogen atmosphere, out-diffusion effect isequivalent to that in hydrogen atmosphere, but natural oxide film on thesurface cannot be removed uniformly. Accordingly, it is preferable toremove the natural oxide film previously before heat treatment with HFaqueous solution. Furthermore, in a mixed atmosphere of argon andnitrogen, out-diffusion effect is equivalent to that achieved inhydrogen atmosphere.

[0062] The heat treatment is conducted at 1100 to 1300° C. for oneminute or more in order to melt sufficiently inner wall oxide film ofthe grown-in defect.

[0063] In heat treatment at high temperature in nitrogen atmosphere,very stable thermal nitride film is formed on the surface of the siliconwafer, and it sometimes takes long time to remove the film in the laterprocesses, or surface roughness on the surface of the wafer is sometimescaused by slight amount of oxygen or water vapor in nitrogen. Theinventors have found that formation of unnecessary film and surfaceroughness on the surface of the wafer caused in nitrogen atmosphere canbe prevented by previously forming a protection oxide film on thesurface of the wafer before heat treatment.

[0064] Furthermore, with such a protection oxide film, in addition tothe effect of protection of the surface of the wafer against formationof unnecessary film and surface roughness, an effect of preventingcontamination with heavy metal impurities diffused inside of the waferfrom the furnace during heat treatment can be achieved.

[0065] Furthermore, a step of melting the inner wall oxide film ofdefect and a step of filling grown-in defects with interstitial siliconare continuously conducted from the following reason. If these two stepsof heat treatment are not conducted continuously, the inner wall oxidefilm of grown-in defect is grown again due to lowering of temperature ofthe wafer, and as a result defects cannot be eliminated or reduced.Therefore, these two steps of heat treatment are continuously conductedwithout cooling to the temperature less than 700° C. at which the innerwall oxide film of grown-in defects begin to grow again.

[0066] In the present invention, a step of filling grown-in defect isconducted by heat treatment in oxidizing atmosphere. Because, accordingto the heat treatment in oxidizing atmosphere, differently from the heattreatment in hydrogen atmosphere, atomic vacancy is never injected, butonly interstitial silicon is injected from the surface of the wafer, sothat grown-in defects can be efficiently filled with interstitialsilicon, and grown-in defects can be eliminated, and surface roughnessand contamination can be prevented by oxidizing the surface activated bythe heat treatment at high temperature in non-oxidizing atmosphere.

[0067] In order to fill and eliminate grown-in defects sufficiently, theheat treatment is preferably conducted at 1000° C. to 1300° C. for oneminute or more. However, at temperature of 700° C. or more, reduction ofgrown-in defects and prevention of surface roughness can be achieved.

[0068] As the oxidizing atmosphere, atmosphere containing water vapor,dry oxygen (dry O₂) 100% atmosphere, or a mixed gas of dry oxygen andargon or nitrogen atmosphere or the like can be adopted.

[0069] Using atmosphere containing water vapor, an oxidation rate ishigh, so that interstitial silicon can be injected efficiently in veryshort time to eliminate defects at a relatively low temperature around700° C. In that case, thickness of the oxide film formed on the surfaceis relatively thick, it is suitable for the case that SOI having a thickburied oxide film is produced.

[0070] On the other hand, using dry oxygen atmosphere or a mixed gasatmosphere of dry oxygen and argon or nitrogen, a growing rate of anoxide film is low, thickness of the oxide film formed after heattreatment can be thin. Therefore, it is suitable for the case that theformed oxide film needs to be removed with an aqueous solution of HF orthe like, or the case that the above-mentioned ion implantationdelamination method is used.

[0071] If a growth rate of the oxide film is low and thickness of theformed oxide film is thin, in the case that the mixed gas atmosphere isused or the like, it has been considered that the effect of eliminatingdefects by injecting the interstitial silicon is inferior. The inventorsperformed the following experiments to confirm what oxygen concentrationand thickness of the oxide film are enough for sufficiently eliminatingdefects.

[0072] Annealing in argon 100% atmosphere, at 1200° C., for 40 minuteswas conducted, and then annealing in 6 kinds of mixed gas of argon anddry oxygen each of which has different oxygen concentration (oxygenconcentration 0, 10, 20, 30, 50, 100%) for 20 minutes was conducted.Then, the surface of the wafer was polished with a stock removal of 5μm, and COP having a size of 0.9 μm or more was measured. The resultswere shown in FIGS. 9(a), (b). Polishing with a stock removal of 5 μmwas conducted in order to observe effect of elimination of COP in asurface-layer part of the wafer. FIG. 9(a) shows a relation betweenoxygen concentration in annealing atmosphere and the number of COP. FIG.9(b) shows a relation between thickness of the oxide film formed byannealing and the number of COP.

[0073] As shown in FIG. 9, even if dry oxygen concentration in a mixedgas atmosphere is only about 10%, in the case that the thickness of theoxide film is 20 nm or more, effect equivalent to that in the case ofusing dry oxygen 100% (thickness of oxide film is about 100 nm) can beobtained.

[0074] Furthermore, it has been found that heat treatment in oxidizingatmosphere after the heat treatment in non-oxidizing atmosphere has aneffect of preventing contamination of the wafer from a tube or a boat toa minimum. FIG. 10 shows comparison of change of contamination level ofmetal impurities in the wafer in the case that annealing in argon 100%atmosphere at 1200° C. for 40 minutes is conducted and then heattreatment in an atmosphere of mixed gas of argon and dry oxygen (oxygenconcentration 30%) for 20 minutes is conducted, and in the case thatheat treatment in hydrogen 100% or argon 100% atmosphere at 1200° C. for60 minutes is conducted, in each case different tube is used, and heattreatment was conducted repeatedly. Measurement of contamination levelwas conducted using SPV (Surface Photo Voltage: trade name, a wafercontamination monitoring system) manufactured by SemiconductorDiagnostic Inc. (SDI).

[0075] It is clear that annealing only with hydrogen or argon may causeetching of a tube or a boat, which may lead to sudden degrading ofimpurity level. On the other hand, as for the heat treatment containingannealing in oxidizing atmosphere, oxide film is formed on the surfaceof the wafer and the surface of a boat or a tube, and therefore theoxide film for protection are always formed, and thus it is consideredthat contamination from a tube or a boat can be suppressed to a minimum.

[0076] Defects that can be eliminated by the oxidation heat treatment at700 to 1300° C. are limited to crystal defects that are not exposed onthe surface of the silicon wafer and exist inside of the wafer. Because,elimination of the defects herein is conducted by injecting interstitialsilicon from the surface due to oxidation to fill the void type crystaldefects therewith. Accordingly, the void type crystal defects such asCOP that are exposed on the surface need to be previously eliminated bymigration of silicon atoms on the surface of the wafer by heat treatmentin argon atmosphere or the like before the oxidation heat treatment.However, if surface protection oxide film is formed, migration of thesurface silicon atoms is suppressed, so that COP on the surface may beeliminated insufficiently.

[0077] The inventors thought out a method of eliminating COP on thesurface of the wafer sufficiently by forming a thermal oxide film with athickness of 300 nm or more on the surface of the wafer after theabove-mentioned oxidation heat treatment at 700 to 1300° C., in the caseof previously forming surface protection oxide film before heattreatment in non-oxidizing atmosphere. Because, if thickness of thethermal oxide film on the surface of the wafer after oxidation heattreatment is 300 nm or more, the shape of COP on the surface gets smoothduring a process of growing the thermal oxide film, and substantiallythe same effect as elimination of COP can be achieved. An average sizeof COP on the surface of the wafer is 100 to 200 nm. If an oxide filmhaving a thickness about 300 nm is formed, it is enough to take COP intothe oxide film and eliminate them.

[0078] The oxide film formed by the oxidation heat treatment can beremoved with an aqueous solution of HF or the like.

[0079] Furthermore, the inventors have found that silicon wafer having alarge size and have few grown-in defects can be produced in highproductivity by a method of growing silicon single crystal ingotaccording to Czochralski method with controlling a cooling rate at 1150°C. to 1080° C. to be 2.3° C./min or more, and a method of growingsilicon single crystal ingot in which nitrogen is doped, and that theeffect of eliminating or reducing grown-in defects can be improved bysubjecting the said silicon wafer to the above-mentioned non-oxidizingheating treatment and oxidation heat treatment of the present invention.

[0080] Namely, it is said that grown-in defects aggregates attemperature in the range of 1150° C. to 1080° C. during pulling ofcrystal. Accordingly, if a cooling rate in the temperature range of 1150to 1080° C. is made 2.3° C./min or more and staying time is shorten,size and number of grown-in defect can be controlled.

[0081] It is pointed out that the agglomeration of atomic vacancies insilicon single crystal can be suppressed, if nitrogen is doped in thesilicon single crystal (T. Abe and H. Takeno, Mat. Res. Soc. Symp. Proc.Vol.262,3,1992). It is considered that the effect can be achieved as aresult that vacancy agglomeration process is transited from homogenousnucleus formation to heterogeneous nucleus formation. Accordingly,silicon single crystal having small grown-in defect can be obtained bygrowing the silicon single crystal by CZ method with doping nitrogen,and thus the silicon single crystal wafer can be obtained by processingit. According to the method, it is not always necessary to decreasegrowth rate of the crystal, differently from the conventional method,and thus a silicon single crystal wafer can be produced in highproductivity.

[0082] It is preferable that oxygen concentration in a single crystalingot is 18 ppma or less, when the silicon single crystal ingot is grownby Czochralski method. Because, if oxygen concentration is low as above,growth of crystal defects can be further suppressed, and formation ofoxide precipitates near the surface of the wafer can be prevented.Especially, when nitrogen is doped in single crystal, oxygenprecipitation is accelerated, and thus it is preferable that formationof oxide precipitates near the surface of the wafer is prevented byusing the oxygen concentration in the above-mentioned range.

[0083] In the present invention, control of size and number of grown-indefects with a cooling rate in Czochralski method can be performed bychanging a pulling rate of the crystal. For example, when a certainspecific pulling apparatus is used, a cooling rate achieved with apulling rate of 1.8 mm/min is higher than a cooling rate achieved with apulling rate of 1.0 mm/min by the same apparatus. As other methods,position and structure or the like of members in the furnace of apulling apparatus called hot zone can be changed to control a coolingrate at a temperature of 1150-1080° C.

[0084] The size of grown-in defect can also be controlled by dopingnitrogen impurity while single crystal is grown according to Czochralskimethod. In that case, silicon single crystal ingot in which nitrogen isdoped can be grown by a known method such as disclosed in, for example,Japanese Patent Application Laid-open (kokai) No 60-251190.

[0085] Namely, nitrogen can be doped in a silicon single crystal byplacing nitride previously in the quartz crucible before growing siliconsingle crystal ingot, adding nitride into the silicon melt, or by usingan atmosphere gas containing nitrogen. The doping amount in the crystalcan be controlled by controlling the amount of nitride, concentration ortime of introduction of nitrogen gas.

[0086] As described above, agglomeration of grown-in defects can besuppressed by doping nitrogen when the single crystal is grown accordingto Czochralski method.

[0087] As for the reason for the size reduction of crystal defectsintroduced into silicon when nitrogen is doped in the silicon singlecrystal, atomic vacancy agglomeration process is transited fromhomogenous nucleus formation to heterogeneous nucleus formation asdescribed above.

[0088] Accordingly, the concentration of nitrogen to be doped ispreferably 1×10¹⁰ atoms/cm³ or more, more preferably 5×10¹³ atoms/cm³ ormore, in which ranges the heterogeneous nucleus formation issufficiently caused. Thereby, agglomeration of crystal defects can besufficiently suppressed.

[0089] On the other hand, if nitrogen concentration is more than 5×10¹⁵atoms/cm³, which is solid solubility of nitrogen in silicon singlecrystal, crystallization of the silicon single crystal is inhibited.Therefore, it should not be more than the above concentration.

[0090] In the present invention, it is preferable that oxygenconcentration in the single crystal ingot is 18 ppma or less, when thesilicon single crystal ingot is grown by Czochralski method. Oxygenconcentration contained in the single crystal ingot can be lowered so asto fall in the above range by a conventional method, when a siliconsingle crystal ingot is grown. For example, oxygen concentration can beeasily controlled to fall in the above mentioned range by reducing thenumber of rotation of a crucible, increasing volume of flowing gas,lowering an atmosphere pressure, controlling temperature distributionand convection of a silicon melt or the like.

[0091] Thereby, the silicon single crystal ingot wherein size and numberof grown-in defects are reduced, can be thus obtained according toCzochralski method. After it is sliced according to a general methodwith a cutting machine such as an inner diameter slicer, a wire saw orthe like, it is subjected to processes including chamfering, lapping,etching, polishing and the like to be a silicon single crystal wafer. Ofcourse, such processes are merely examples, and various other processessuch as cleaning or the like can be conducted, and process can bechanged appropriately depending on the purpose, namely, order ofprocesses can be changed, and some processes can be omitted.

[0092] Thereby, CZ silicon single crystal wafer used as a bond wafer inthe present invention can be obtained. A method of producing SOI waferof the present invention using the CZ silicon wafer will be explainedbelow. FIGS. 1(A) to (E) is a flow chart showing one example of theprocess for producing a bonded SOI wafer of the present invention. FIG.2 shows outline of the heat treatment to which the silicon singlecrystal wafer to be a bond wafer is subjected before bonding.

[0093] First, CZ silicon single crystal wafer 5 that is to be a bondwafer is subjected to heat treatment consisting of two steps shown inFIGS. 1(B) and (C) and FIG. 2. First, as the first step, annealing isconducted in a temperature range of 1100° C. to 1300° C. in 100% Ar gasatmosphere for more than one minute to out-diffuse oxygen in the crystaland melt oxide film inner wall of void defects. Thereby, low-defectlayer 3 is formed in the silicon single crystal wafer 5 (FIG. 1(B), FIG.2). Then, annealing is conducted continuously in an oxidizing atmosphereat a temperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to form an oxidefilm 4, to inject interstitial silicon from an interface of Si/SiO₂, andeliminate void defects to a deeper part of the crystal, and enlarge thelow-defect layer 3 (FIG. 1(C), FIG. 2). According to the method, COP canbe effectively eliminated from the surface to the depth of about 5 to 10μm or more.

[0094] The heat treatment can be conducted using any types of heattreatment furnace commercially available, as far as it is a heattreatment furnace wherein cleanness is controlled.

[0095] For example, a heater heating horizontal type or vertical typediffusion furnace can be used, or a lamp heating type single waferprocessing wafer heating apparatus can be used. It is important thatsufficient heat treatment temperature and heat treatment time innon-oxidizing atmosphere and heat treatment temperature and heattreatment time for the subsequent heat treatment in oxidizing atmosphereare ensured in order to eliminate or reduce grown-in defectseffectively, and that two heat treatments are conducted continuously sothat temperature is not lowered too much.

[0096] Therefore, it is necessary to subject silicon single crystalwafer 5 to heat treatment in a non-oxidizing atmosphere, especiallyargon, nitrogen or mixed gas of argon and nitrogen at a temperature of1100 to 1300° C. for one minute or more, and then to subsequentoxidation in an oxidizing atmosphere at a temperature of 700° C. to1300° C. for one minute or more without cooling the wafer to atemperature less than 700° C.

[0097] As described above, if the heat treatment in a non-oxidizingatmosphere, especially argon, nitrogen or mixed gas of argon andnitrogen and the subsequent oxidation heat treatment are not conductedcontinuously, the inner wall oxide film of grown-in defects are grown,and as a result the defects cannot be eliminated or reduced.Accordingly, it is preferable to conduct continuously the heat treatmentin an atmosphere of argon or the like, and subsequent oxidation heattreatment, before the wafer 5 is cooled to the temperature less than700° C., without taking out the wafer 5 from the furnace. Furthermore,heat treatment time can be shorten, since the heat treatments areconducted continuously at the same temperature.

[0098] In order to conduct the heat treatment as above, after heattreatment in an atmosphere of argon, nitrogen or a mixed gas of argonand nitrogen or the like is conducted, the atmosphere gas was exhausted,and oxygen gas is subsequently introduced at desired concentration toconduct the oxidation heat treatment. According to the presentinvention, since the initial heat treatment for melting an inner walloxide film of defects is conducted in non-oxidizing gas atmosphere suchas argon, nitrogen or a mixed gas of argon that contains hydrogen in anamount less than explosion limit (about 4%), the next oxidation heattreatment can be safely even if a conventional commercially availablefurnace is used.

[0099] Furthermore, in the case that a protection oxide film ispreviously formed on the surface of the wafer that is to be subjected tothe heat treatment for melting of inner wall oxide film of defects innon-oxidizing atmosphere such as argon, nitrogen or a mixed gas of argonand nitrogen, the heat treatment for forming the above oxide film can beconducted continuously before the heat treatment for melting an innerwall oxide film, or it can be conducted previously as an independentstep. The oxide film may be formed, for example, by thermal oxidationsuch as so-called dry oxidation using dry oxygen, wet oxidationcontaining water vapor, or CVD oxide film formed by CVD (Chemical VaporDeposition) method.

[0100] Heat treatment in oxidizing atmosphere as the second step shownin FIG. 1(C) and FIG. 2 can be conducted by either dry oxidation in anatmosphere containing no water vapor or wet oxidation in an atmospherecontaining water vapor. In both the ways above, equivalent effect can beexpected as for injection of interstitial silicon into grown-in defectsand improvement in surface roughness that are essential objects of thepresent invention.

[0101] Then, as shown in FIG. 1(D), a bonded SOI wafer is produced usingthe above silicon single crystal wafer on which the silicon oxide filmis formed as a bond wafer 1. As shown in FIG. 1(D), since the oxide film4 formed in the preceding step is used as BOX of SOI wafer, steps can besimplified. Furthermore, since an oxide film is formed afterAr-annealing, BOX can be formed excellent in film quality. The wafer isbrought in close contact with a base wafer 2 via the BOX at roomtemperature, and subjected to the bonding heat treatment at 200° C. ormore, generally at about 1000° C. to 1200° C. As the base wafer 2, asilicon single crystal wafer is generally used, but insulator substrate(quartz, sapphire or the like) can also be used depending on the use.Furthermore, in the case that a silicon single crystal wafer is used, itcan be bonded to a base wafer 2 after an oxide film is formed on thebase wafer.

[0102] After conducting the bonding heat treatment, SOI wafer 10 isproduced by conducting general process for decreasing thickness such asgrinding, polishing or the like (FIG. 1(E)). Thereby, there can beobtained the bonded SOI wafer 10 wherein BOX 12 consisting of the oxidefilm 4 and the SOI layer 11 consisting of the low-defect layer 3 areformed on the base wafer 2. Since the SOI layer 11 of the SOI wafer 10consists of the low-defect layer 3, there are very few defects such asCOP at any region in depth direction. In that case, a vapor phaseetching called PACE (Plasma Assisted Chemical Etching) can be conductedto decrease thickness of the bond wafer 1 (Japanese Patent No. 256567).

[0103] In the case that SOI wafer is produced using ion implantationdelamination method (technique called smart-cut, Japanese PatentApplication Laid Open (Kokai) No. 5-211128), an oxide film 4 is formedon the surface of the silicon single crystal wafer 5 by theabove-mentioned two step heat treatment, and then hydrogen ions or raregas ions are implanted through the oxide film 4, and it is used as abond wafer 1, which is then bonded to the base wafer 2.

[0104] In that case, since deviation in thickness of the formed SOIlayer is a total of deviation in depth of ion implantation and deviationof thickness of the oxide film, in order to decrease it as possible, itis desirable to decrease thickness of the oxide film 4 formed on thesilicon single crystal wafer 5 that is to be a bond wafer 1 as possibleto decrease an absolute value of deviation in thickness of the oxidefilm prepared. Therefore, thickness of the oxide film is preferably 100nm or less, and preferably 20 nm or more in order to obtain effect ofeliminating defects.

[0105] In the case that thickness of the oxide film formed on the bondwafer is 100 nm or less as described above, and a thicker BOX of SOIwafer is necessary from the point of device design, it may be bonded tothe base wafer after the lacking oxide film is formed.

[0106] Furthermore, the bond wafer that is delaminated when SOI wafer isproduced according to such a hydrogen ion implantation delaminationmethod can be used as a new bond wafer or a base wafer. As describedabove, the bond wafer produced as a byproduct after delamination has adenuded zone with sufficient depth in a surface-layer part, and containsa sufficient amount of oxide precipitate precipitated in a bulk part dueto heat treatment, so that it can be a good bond wafer or a good basewafer.

[0107] In that case, one surface of the wafer produced as byproduct inthe present invention is a delaminated surface, and the surface of theother side is a plain surface of original silicon wafer. Accordingly,treatment such as grinding, polishing or the like has to be applied onlyon the delaminated surface. Accordingly, the process is simple sinceonly one surface is treated, and stock removal is slight. Namely, when asilicon wafer is provided by slicing a general silicon ingot, bothsurfaces are cut surface, so that steps such as a lapping step, anetching step or the like are necessary, and the stock removal is large.However, the delaminated wafer of the present invention has a plainsurface on one side, and therefore only the delaminated surface has tobe ground or polished on the basis of the plain surface, and the sameplain surface as a general silicon mirror surface wafer can be obtainedwith a slight stock removal.

[0108] If the silicon wafer obtained by reprocessing the delaminatedwafer is reused as a bond wafer or a base wafer of a SOI wafer,substantially one SOI wafer can be obtained from one silicon wafer, sothat silicon wafer can be used effectively as material.

EXAMPLES

[0109] The following examples and comparative examples are beingsubmitted to further explain the present invention. These examples arenot intended to limit the scope of the present invention.

Example 1, Comparative Example 1, Comparative Example 2

[0110] In accordance with the method of the present invention, a bondwafer of a bonded SOI wafer was produced, and quality thereof wasevaluated. As a silicon single crystal wafer that is to be a bond wafer,a wafer sliced from 8″φCZ silicon single crystal having orientation<100>, interstitial oxygen concentration of 16 ppma (JEIDA) pulled witha pulling rate of 1.2 mm/min was used.

[0111] The wafer was subjected to the heat treatment of the presentinvention. Annealing was conducted at 1200° C. for 40 minutes in anatmosphere of 100% Ar using VERTEX3(DD-813V) manufactured by KokusaiElectric Co., Ltd., as an annealing furnace, and subsequently annealingat the same temperature in an atmosphere of a mixed gas of 30% oxygenand 70% argon for 20 minutes was conducted. An oxide film having athickness of about 30 nm was formed.

[0112] The wafer subjected to annealing was then subjected to treatmentfor removing an oxide film with a solution of hydrofluoric acid, andthen to polishing with a stock removal of 5 μm, and then the number ofCOP (size of 0.09 μm or more) at a deep zone was measured. Measurementof COP was conducted using SurfScan SP1 manufactured by KLA TencorCorporation. For comparison, a wafer obtained by subjecting the samesilicon single crystal wafer as above to annealing of H₂/1200° C./onehour (Comparative example 1) and a wafer obtained by subjecting the samewafer to annealing of Ar/1200° C./one hour (Comparative example 2) werepolished with a stock removal of 5 μm, and the number of COP wasmeasured in a similar method to the above.

[0113] The result of measurement was shown in FIG. 3. As shown in FIG.3, the number of COP in the wafer of Example 1 was 400 or less in a8-inch wafer, which corresponded to COP density of 1.3/cm² or less.Accordingly, the method of the present invention had a higher grown-indefect eliminating effect compared with conventional H₂- orAr-annealing.

[0114] Oxide dielectric breakdown voltage of the wafers polished with astock removal of 5 μm was measured. The results were shown in FIG. 4 andFIG. 5. A good chip yield of TDDB (Time Dependent Dielectric Breakdown)herein means a good chip yield when a chip having a time dependentdielectric breakdown voltage of 25 C/cm² or more measured under thecondition: gate oxide film thickness of 25 nm, gate area of 4 mm²,stress electric current of 0.01 A/cm² and room temperature, or a chiphaving a time dependent dielectric breakdown voltage of 5 C/cm² or moremeasured under the condition: gate oxide film thickness of 25 nm, gatearea of 4 mm², stress electric current of 0.01 A/cm² and 100° C., isdefined as a good chip.

[0115] A good chip yield of TZDB (Time Zero Dielectric Breakdown) hereinmeans a good chip yield when a chip having a time zero dielectricbreakdown voltage of 8 MV/cm or more measured under the condition: gateoxide film thickness of 25 nm, gate area of 8 mm², 1 mA/cm² of electriccurrent density in decision at room temperature is defined as a goodchip.

[0116] As shown in FIGS. 4 and 5, the wafer subjected to the heattreatment of the present invention had excellent oxide dielectricbreakdown voltage even in a deep zone, compared to the wafers subjectedto H₂- or Ar-annealing.

[0117] From the above results, it is clear that if a silicon singlecrystal that is to be a bond wafer is produced according to the methodof the present invention, the wafer having few crystal defects andexcellent oxide dielectric breakdown voltage can be produced.Accordingly, if a SOI wafer is produced using such a silicon singlecrystal wafer, SOI wafer having few crystal defects can be obtained.

[0118] The SOI wafers having SOI layer with a thickness of about 0.1 μmwere produced according to an ion implantation delamination method,using three kinds of bond wafers produced under the above-mentionedcondition. Production condition was as follows.

[0119] 1) Hydrogen ion implantation condition:

[0120] H+ion, implantation energy 30 keV

[0121] 2) Delamination heat treatment condition:

[0122] oxidizing atmosphere, 500° C., 30 minutes

[0123] 3) Bonding heat treatment condition:

[0124] nitrogen atmosphere (containing slight amount of oxygen), 1200°C., 120 minutes

[0125] 4) Touch polishing (polishing with slight stock removal on thesurface of SOI) done

[0126] 5) Base wafer oxide film: 300 nm

[0127] COP in SOI wafer produced above was observed according to a HFdip method. The HF dip method comprises dipping the SOI wafer having thethin SOI layer as above was dipped in a 50% aqueous solution of HF, ifthere is a defect penetrating the SOI layer, HF reaches BOX through itto etch the oxide film and form etch pits, and observing etch pits witha microscope through the thin SOI layer to evaluate COP in a wafer. Theresults were shown in Table 1. TABLE 1 Heat treatment COP densityatmosphere (number/cm²) Example 1 Ar + Ar/O₂ 0.2 Comparative only H₂ 1.8Example 1 Comparative only Ar 1.9 Example 2

[0128] As shown in Table 1, SOI wafer of Example contains very few COPpenetrating the SOI layer, compared with conventional SOI wafersobtained by subjecting bond wafers only to H₂-annealing or Ar-annealing.As described above, the density of COP having a size of 0.09 μm or morein the SOI wafer of Example 1 was 1.3/cm² or less at any depth in theSOI layer. Namely, SOI wafer having quite excellent quality can beobtained.

[0129] Although there remained a step of about 0.2 to 0.3 μm in aperipheral part of the bond wafer after delamination formed as byproductwhen SOI wafer of Example 1 was produced, the step could be removed onlyby removing the oxide film on the surface, and then polishing thedelaminated surface with a stock removal of about 1 μm, and good mirrorsurface having no exposed oxide precipitates could be obtained.Accordingly, it was confirmed that there was no problem in bonding evenwhen the wafer was used as a new bond wafer or a base wafer.

Example 1, Example 2 and Example 3

[0130] In accordance with the method of the present invention, a bondwafer of a bonded SOI wafer was produced using three kinds of siliconsingle crystal wafer, and quality thereof was evaluated. As siliconsingle crystal wafers, the wafer used in Example 1, the wafer slicedfrom a single crystal produced under the same condition as Example 1except that a pulling rate of the single crystal was 1.9 mm/min (Example2), and the wafer sliced from a single crystal produced under the samecondition as Example 1 except that the crystal in which nitrogen wasdoped at 10¹⁴ atoms/cm³. These wafers were subjected to annealing at1200° C. for 40 minutes in an atmosphere of 100% Ar, and then annealingat the same temperature in an atmosphere of a mixed gas of 30% oxygenand 70% argon for 20 minutes. After removing an oxide film with a HFsolution, and the wafers were polished with a stock removal of 5 μm, andthen the number of COP (≧0.09 μm) at a deep zone was measured. Theresults were shown in FIG. 6.

[0131] As shown in FIG. 6, the crystal containing the least COP was thewafer consisting of the crystal in which nitrogen was doped, and thenumber of COP gets large in the order of the above-mentioned wafer, thewafer pulled at high speed, and the wafer pulled at general speed.Accordingly, a bond wafer having less grown-in defects can be producedusing the crystal pulled at high speed or the crystal in which nitrogenwas doped. When the crystal was pulled at high speed, time for pullingcrystal can be shorten, and thus throughput can be improved.

[0132] As for the bond wafers produced under three kinds of condition asdescribed above, SOI wafers were produced by the same method as Example1, and COP was evaluated. The results were shown in Table 2. TABLE 2 COPdensity Bond wafer (number/cm²) Example 1 General pulling rate 0.2 nonitrogen was doped Example 2 High pulling rate 0.1 no nitrogen was dopedExample 3 General pulling rate  0.01 nitrogen was doped

[0133] As shown in Table 2, COP density of the SOI wafer produced usingthe silicon single crystal wafer consisting of crystal pulled at highspeed was half of COP density of the SOI wafer produced using a generalsilicon single crystal wafer. COP density of the SOI wafer producedusing the silicon single crystal wafer consisting of crystal in whichnitrogen was doped was one twentieth of general one. Accordingly, theSOI wafer having further excellent SOI layer can be obtained using thecrystal pulled at high speed or the crystal in which nitrogen was doped.

Example 4, Comparative Example 3, Comparative Example 4

[0134] The same silicon single crystal wafer as used in Example 1 wassubjected to annealing in 100% Ar atmosphere at 1200° C. for 40 minutes,and subsequently to oxidation in atmosphere containing water vapor at150° C. for 240 minutes to form an oxide film having a thickness of 1.0μm. The bonded SOI wafer having SOI layer with thickness of 5 μm and BOXlayer with thickness of 1 μm was produced according to a generalgrinding and polishing method using the wafers produced above as a bondwafer. Thickness of the oxide film was measured using MPV-SPmanufactured by Leitz Corporation.

[0135] Oxide dielectric breakdown voltage was compared as for the SOIwafer produced above, the SOI wafer subjected to annealing of H₂/1200°C./one hour (Comparative Example 3) or the wafer subjected to annealingof Ar/1200° C./one hour (Comparative Example 4), then cooled to roomtemperature, and subsequently subjected to oxidation heating treatment(oxidation in atmosphere containing water vapor at 1150° C. for 240minutes). Measurement condition was the same as Example 1.

[0136] The results were shown in FIG. 7 and FIG. 8. As shown in theresults, the wafer subjected to the heat treatment of the presentinvention has excellent in TZDB and TDDB, even when the thickness of theSOI layer was more than 0.5 μm, namely has excellent in oxide dielectricbreakdown voltage, whereas a conventional annealing method is noteffective for the wafer having the SOI layer with the same thickness asabove.

[0137] The present invention is not limited to the above-describedembodiment. The above-described embodiment is a mere example, and thosehaving the substantially same structure as that described in theappended claims and providing the similar action and effects areincluded in the scope of the present invention.

[0138] For example, when silicon single crystal ingot is grown accordingto Czochralski method whether nitrogen is doped or not, a magnetic fieldmay be applied to a melt or not. Namely, the term “a Czochralski method”includes not only general Czochralski method but also MCZ method.

[0139] Furthermore, heat treatment at high temperature in non-oxidizingatmosphere and heat treatment in oxidizing atmosphere that is essentialfeature of the present invention can be applied to any steps inprocessing of a wafer. For example, it can be applied after chemicaletching step after cutting a wafer, or after rough polishing step thatis following to the above step, or after the final polishing step or thelike.

[0140] As for the heat treatment in non-oxidizing gas atmosphere in thepresent invention, explanation in the above embodiment of the presentinvention has focused on the case that argon or nitrogen is used.However, atmosphere is not limited to argon and nitrogen, there can beused the above-gas containing hydrogen in an amount less than explosionlimit, rare gas having the same effect as argon, such as helium, neon,krypton, xenon can be used, and are included in the scope of the presentinvention.

1. A method of producing a bonded SOI wafer comprising bonding a bondwafer and a base wafer via an oxide film and then reducing thickness ofthe bond wafer, wherein a silicon single crystal ingot is grownaccording to Czochralski method, the single crystal ingot is then slicedto produce a silicon single crystal wafer, the silicon single crystalwafer is subjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, and the resultant wafer is used as the bond wafer.
 2. Amethod of producing a bonded SOI wafer comprising bonding a bond waferand a base wafer via an oxide film and then reducing thickness of thebond wafer, wherein a silicon single crystal ingot is grown according toCzochralski method, the single crystal ingot is then sliced to produce asilicon single crystal wafer, the silicon single crystal wafer issubjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, at least one of hydrogen ions and rare gas ions areimplanted into the surface via a silicon oxide film of the wafer to forman ion implanted layer, and the resultant wafer is used as the bondwafer, which is then brought into close contact with the base wafer viathe silicon oxide film of the bond wafer, followed by delamination atthe ion implanted layer by heat treatment.
 3. A method of producing thebonded SOI wafer wherein the bond wafer delaminated at the ion implantedlayer in the method of producing a bonded SOI wafer described in claim 2is used as a new bond wafer.
 4. A method of producing the bonded SOIwafer wherein the bond wafer delaminated at the ion implanted layer inthe method of producing a bonded SOI wafer described in claim 2 is usedas a new base wafer.
 5. The method of producing the bonded SOI waferaccording to claim 1 wherein the non-oxidizing atmosphere is argon,nitrogen or a mixed gas of argon and nitrogen.
 6. The method ofproducing the bonded SOI wafer according to claim 2 wherein thenon-oxidizing atmosphere is argon, nitrogen or a mixed gas of argon andnitrogen.
 7. The method of producing the bonded SOI wafer according toclaim 1 wherein the oxidizing atmosphere is atmosphere containing watervapor.
 8. The method of producing the bonded SOI wafer according toclaim 2 wherein the oxidizing atmosphere is atmosphere containing watervapor.
 9. The method of producing the bonded SOI wafer according toclaim 1 wherein the oxidizing atmosphere is dry oxygen atmosphere or amixed gas atmosphere of dry oxygen and argon or nitrogen.
 10. The methodof producing the bonded SOI wafer according to claim 2 wherein theoxidizing atmosphere is dry oxygen atmosphere or a mixed gas atmosphereof dry oxygen and argon or nitrogen.
 11. The method of producing thebonded SOI wafer according to claim 1 wherein thickness of the oxidefilm formed by the heat treatment in the oxidizing atmosphere is 20 to100 nm.
 12. The method of producing the bonded SOI wafer according toclaim 2 wherein thickness of the oxide film formed by the heat treatmentin the oxidizing atmosphere is 20 to 100 nm.
 13. The method of producingthe bonded SOI wafer according to claim 9 wherein thickness of the oxidefilm formed by the heat treatment in the oxidizing atmosphere is 20 to100 nm.
 14. The method of producing the bonded SOI wafer according toclaim 10 wherein thickness of the oxide film formed by the heattreatment in the oxidizing atmosphere is 20 to 100 nm.
 15. The method ofproducing the bonded SOI wafer according to claim 1 wherein the oxidefilm is previously formed on the surface of the wafer before the heattreatment in a non-oxidizing atmosphere.
 16. The method of producing thebonded SOI wafer according to claim 2 wherein the oxide film ispreviously formed on the surface of the wafer before the heat treatmentin a non-oxidizing atmosphere.
 17. The method of producing the bondedSOI wafer according to claim 15 wherein thickness of the thermal oxidefilm on the surface of the wafer after the above-mentioned heattreatment in the oxidizing atmosphere is 300 nm or more.
 18. The methodof producing the bonded SOI wafer according to claim 16 whereinthickness of the thermal oxide film on the surface of the wafer afterthe above-mentioned heat treatment in the oxidizing atmosphere is 300 nmor more.
 19. The method of producing the bonded SOI wafer according toclaim 1 wherein a silicon single crystal ingot is grown according toCzochralski method with controlling a cooling rate at 1150° C. to 1080°C. of the single crystal ingot to be 2.3° C./min or more.
 20. The methodof producing the bonded SOI wafer according to claim 2 wherein a siliconsingle crystal ingot is grown according to Czochralski method withcontrolling a cooling rate at 1150° C. to 1080° C. of the single crystalingot to be 2.3° C./min or more.
 21. The method of producing the bondedSOI wafer according to claim 1 wherein a silicon single crystal ingot inwhich nitrogen is doped is grown according to Czochralski method. 22.The method of producing the bonded SOI wafer according to claim 2wherein a silicon single crystal ingot in which nitrogen is doped isgrown according to Czochralski method.
 23. The method of producing thebonded SOI wafer according to claim 21 wherein silicon single crystalingot in which nitrogen is doped according to Czochralski method, andthe concentration of nitrogen doped in the single crystal ingot is1×10¹⁰ to 5×10¹⁵ atoms/cm³.
 24. The method of producing the bonded SOIwafer according to claim 22 wherein silicon single crystal ingot inwhich nitrogen is doped according to Czochralski method, and theconcentration of nitrogen doped in the single crystal ingot is 1×10¹⁰ to5×10¹⁵ atoms/cm³.
 25. The method of producing the bonded SOI waferaccording to claim 1 wherein the concentration of oxygen contained inthe single crystal ingot is 18 ppma or less.
 26. The method of producingthe bonded SOI wafer according to claim 2 wherein the concentration ofoxygen contained in the single crystal ingot is 18 ppma or less.
 27. Abonded SOI wafer produced by the method according to claim
 1. 28. Abonded SOI wafer produced by the method according to claim 2.